In the following specification, the term "ribbon" designates an element of indefinite length, of small width, for example some millimeters to some centimeters, constituted by a multi-filament yarn, or a lap of multi-filament yarns, impregnated with a polymer performing the role of matrix. This type of ribbon serves as the base for making composite materials, it is shaped for example by weaving, knitting, braiding, and the final composite product is obtained by a subsequent heat treatment: pultrusion, filament winding, compression.
Most composite materials with organic matrix are elaborated from yarns embedded in a thermosettable resin; the yarns are principally made of glass, carbon or aramide, the resins are principally epoxides, polyesters, phenolic resins and polyurethanes. These thermosettable resins are employed in solution or in the form of monomers, with a low viscosity, which ensures a very good impregnation of each filament constituting the reinforcing yarns. These composite materials have good mechanical properties both in elongation and in shear. However, they present a drawback from the mechanical standpoint, namely a certain fragility to shock and, furthermore, taking into account the kinetics of cross-linking of the thermosettable resins employed, the time necessary for making them is long.
It has already been sought to overcome these drawbacks by making composite materials from resins which are no longer thermosettable but thermoplastics. In fact, the use of these resins is simpler, their resistance to shock and fatigue resistance are better. However, in the molten state, they have a high viscosity which prevents a thorough penetration of the reinforcing yarns and therefore a homogeneous coating by the resin of the multi-filaments constituting each reinforcing yarn. In certain limited cases, allowing particular technical means to be carried out, a good thorough penetration may be obtained by working at high pressures; this is the case of so-called injection and compression techniques.
For the other techniques, for example filament winding, pultrusion or pulforming, the quality of the composite pieces obtained depends essentially on the quality of the thorough and homogeneous impregnation of the reinforcing yarns.
French Patents Nos. 74 01975, 83 03938 and 81 07971, disclose processes for obtaining a good impregnation of the reinforcing yarns by the thermoplastics resin. According to these processes, the thermoplastics polymer is in the form of a powder which is made to penetrate in the yarn by different techniques (electrostatic, suspension, fluidized bed). This powder must have a well controlled granulometry centred around 10 to 15 microns. In French Patent No. 84 05627, the yarn of which the filaments are surrounded by the thermoplastics powder, is then coated with a sheath having for its effect to block the powder inside the yarn and therefore avoid deterioration of the yarn when the composite product is made.
Application of these new processes is restricted, since not many thermoplastics polymers are in the form of calibrated powders.
An article published in the German Journal KUNST-STOFFE No. 2 in 1988, discloses that multi-filament yarns may be impregnated with molten thermoplastics materials by passing them in an impregnation chamber. As illustrated in FIG. 9 of this Article, after impregnation, the multifilament yarns are taken along by extraction and cooling rollers.
According to Applicant, the process thus described does not allow a homogeneous penetration of the thermoplastics material, necessary for a perfect cohesion of the filaments and of the thermoplastics material.